1. Field of the Invention
The present invention relates to method for monitoring the buildup of films of microorganisms on surfaces in aquatic systems, such as in heat exchangers, waste water transport and secondary oil recovery, particularly in recirculating water systems such as in cooling systems. In particular the invention relates to a method which is more easily carried out than known methods of this type. The invention also relates to the detection and monitoring of pathogenic bacteria in association with such films.
2. Brief Description of the Prior Art
In all industrial, domestic and medical aquatic systems there are many factors which can create problems by reducing the efficiency of the process concerned or by creating toxicity problems. Water used in most of these systems will usually contain some form of living microorganisms which under the conditions in the systems concerned can multiply and cause a number of problems. Initially the microbes are introduced suspended in the water. These so-called planktonic microorganisms are relatively easy to detect and monitor and also to kill using conventional biocides. Although they can cause problems, for instance if the microorganisms are pathogenic, in general they do not significantly affect the overall efficiency of the system.
The planktonic microorganisms can however become adherent to internal surfaces in the system when they are known as "sessile". Sessile bacteria can proliferate and some bacteria generate so called slime composed mainly of polysaccharide and form a film upon the surface. These films have recently been recognised as contributing to inefficiencies in the systems for various reasons. For instance the films will reduce efficiency of conductive heat transfer through the surfaces and, since they are highly elastic, increase fluid frictional resistance at the surface dramatically. Furthermore some types of bacteria produce compounds which may be environmentally hazardous or may lead to corrosion of metal in the surface to which the film is attached. Sulphate reducing bacteria or SRB's in particular can give rise to serious corrosion of metal surfaces. Of the environmentally hazardous bacteria, Legionella bacteria are known to occur widely in natural and man-made aqueous systems. L. pneumophila are the most common cause of Legionella related diseases in man, and also appears to be the most common species in a man-made environment. The pathogenic nature of Legionella bacteria means that their detection and monitoring is particularly important.
Bacteria in biofilms are in general found to be difficult to get rid of, partly because chemical biocides must penetrate the slime before they reach the target microorganisms deep within the films. Because of the problems that biofilms can create, it is important to be able to detect the appearance of the biofilms in order to know when a dose of a suitable biocide is necessary to prevent development of films and to get rid of existing films, as well as to be able to detect the removal of such films after treatment with such a biocide. Since films which are so thin as to be invisible to the naked eye can cause severe problems it is desirable to be able to detect films by other means. The absence of significant numbers of planktonic bacteria does not necessarily indicate the absence of sessile bacteria. For example, studies have shown that drinking water can be colonised with microorganisms despite treatment of the source water to standards set by the EEC and WHO guidelines. It has been suggested ("A Continuous Culture Biofilm Model for the study of Medical and Industrial Corrosion", Keevil et al) that the transient appearance of pathogens such as Legionella in these waters might indicate that biofilms provide protected micro environments for microbial survival.
In addition, Lechevallier et al (1988) reported that the concentration X time product required to kill 99% of the heterotrophic bacteria in a biofilm with HOCl was 150-3000.times.greater than for the same organisms freely suspended in the water.
With regard to Legionella bacteria, it has been recorded subsequent to the priority date of this application in Journal of Applied Bacteriology Symposium Supplement 1991, 70, 121S-129S (Lee and West) that the control measures for the prevention of Legionella bacteria in aquatic systems must be targeted at the prevention of biofilm development.
Thus, the importance of the provision of an effective method for monitoring the presence and development of biofilm in aquatic systems has been confirmed. In the case of pathogens such as Legionella bacteria, the link between transient appearances of pathogenic planktonic bacteria and biofilms emphasises the particular importance of biofilm detection and monitoring.
One type of device for monitoring biofilm buildup is described in the Canadian Journal of Microbiology (1981), volume 27, pages 910 to 917, in which McCoy et al describe the use of a so-called Robbins device which comprises a tube through which water in a recycling circuit can flow. The tube has a plurality of ports in its walls, each being provided with a stud having a biofoulable surface and being capable of being retained in the port in fixed relationship with respect to the tube so that the biofoulable surface forms part of the internal surface of the tube. The studs may be removed from the ports after a desired time interval and the test surfaces by microscopy of the surfaces analysed for the growth of microorganisms or by removal of the microorganisms from the surfaces and subsequent estimation of the degree of growth. The number of microorganisms can be estimated for instance by physical or chemical means, e.g. by detection of bacterial ATP or by further culturing the microorganisms and analysing the products.
One problem with conventional Robbins devices is that it is difficult to remove the studs from their retaining means for analysis of the biofilm and, having removed the stud, it is difficult to handle such a small component aseptically to avoid cross-contamination.
Ruseska et al in Oil and Gas Journal, March 8th 1982 describe the removal of biofilm bacteria from the test surfaces of a Robbins device by scraping with a sterile scalpel. It is difficult to remove all traces of the film from the surface using a scalpel. In "Developments in Industrial Biology" (1982) chapter 53, McCoy and Costerton describe the removal of biofilm from studs by dropping the entire stud into a test tube containing water and metallic tumbling abrasive. The tube is vortexed for two minutes to abrade the film from the surface. A problem with the latter form of removal of biofilm is that film from other areas of the stud is also removed into the same water. This can lead to erroneous results since bacteria growing on such surfaces do not necessarily simulate growth on the internal surfaces of the rest of the recirculating water system, the conditions of flow and other apsects of their environment possibly being significantly different. It is practically impossible to prevent growth occurring on such surfaces but it would be desirable to be able to sterilise such surfaces without affecting the film on the test surface before its analysis.
Accordingly all the present forms of apparatus are difficult to handle or give results that are inevitably inaccurate, or both.
In U.S. Pat. No. 3174332 an apparatus described as a Test Coupon Positioner is described which tests the effects of coal slurries pumped under pressure on metallic "coupons". The device allows coupons to be positioned in the bulk fluid flow within a pipeline and to be removed after predetermined periods for investigation of any corrosion or erosion of the coupon. The coupons may be removed by use of a removal member which has mounting means for connection with the coupon positioner.
However, the provision of a coupon in the bulk flow of the liquid would be an ineffective method of detecting and monitoring biofilm formation on internal surfaces of the aquatic system because the liquid flow characteristics, past the surface, affect biofilm build-up. Therefore, since bulk flow is not the same as liquid flow characteristics past the internal surfaces of the system, such a coupon would not be effective as a biofilm monitor for internal surfaces as it is not representative of such surfaces.